Depositional and Structural Evolution of a Foreland Basin Margin in a Magnetostratigraphic Framework: the Eastern Swiss Molasse Basin

Home , Alpine Foreland, Austroalpine nappes, Flysch, Helvetic (geology), Helvetic nappes, Infrahelvetic complex

Int Journ Earth Sciences (1999) 88:253–275 Q Springer-Verlag 1999

ORIGINAL PAPER

O. Kempf 7 A. Matter 7 D. W. Burbank 7 M. Mange Depositional and structural evolution of a foreland basin margin in a magnetostratigraphic framework: the eastern Swiss Molasse Basin

Received: 11 March 1998 / Accepted: 12 March 1999

Abstract This integrated study of the sedimentology, Key words Molasse Basin 7 Alpine tectonics 7 magnetostratigraphic chronology and petrography of Alluvial sedimentation 7 Magnetostratigraphy 7 the mostly continental clastics of the Oligocene to Foreland basin evolution Miocene Swiss Molasse Basin underpins a reconstruc- tion of facies architecture and delineates relationships between the depositional evolution of a foreland-basin Introduction margin and exhumation phases and orogenic events in the adjacent orogen. A biostratigraphically based high- The Molasse Basin is a classical foredeep at the resolution magnetostratigraphy provides a detailed northern side of the Alps. Displaying a highly asymme- temporal framework and covers nearly the whole strati- trical cross section with a thick clastic succession, it graphic record of the Molasse Basin (31.5–13 Ma). exceeds 4 km in the south and feathers out towards the Three transverse alluvial fan systems evolved at the north (Fig. 1). It is widely accepted that the structural southern basin margin. They are characterized by development of the orogenic wedge controls the geom- distinct petrographic compositions and document the etry and shape of the foreland basin and therefore has exhumation and denudation history of the growing strong influence on the facies distribution within the eastern Swiss Alps. Enhanced northward propagation basin (Beaumont 1981; Jordan 1981); hence, the strati- of the orogenic wedge is interpreted to have occurred graphy of a foreland basin reflects the tectonic evolu- between 31.5 and 26 Ma. During the period 24–19 Ma, tion of the adjacent orogen. intense in-sequence and out-of-sequence thrusting took The stratigraphy and the facies relationships of the place as Molasse strata were accreted to the orogenic Molasse have been studied intensely over the past wedge. A third active tectonic phase, possibly caused decades (e.g., Homewood and Allen 1981; Allen et al. by backthrusting of the Plateau Molasse, probably 1985; Berger 1985; Diem 1986; Keller 1989; Schoepfer occurred between ca. 15 and 13 Ma. Fan head migra- 1989; Schlunegger et al. 1993), but the causal linkages tion between 31.5 and 13 Ma is probably controlled by between the tectonic evolution of the Alps and the stra- the structural evolution of the thrust front due to tigraphic record in the basin (Homewood et al. 1986; Molasse accretion and backthrusting. Pfiffner 1986; Sinclair et al. 1991; Sinclair and Allen 1992) have remained obscure due to uncertainties in the chronological record. Now a newly established high-resolution magnetostratigraphy in the Swiss Molasse Basin provides a very detailed chronostrati- O. Kempf (Y) 7 A. Matter graphic framework (Burbank et al. 1992; Schlunegger Geologisches Institut, Universität Bern, Baltzerstrasse 1, et al. 1996; Kempf et al. 1997). Based on this, Schlun- CH-3012 Bern, Switzerland egger et al. (1997b) have established a close temporal e-mail: oliver.kempf6geo.unibe.ch, Tel.: c41-31-6318759, link of the relationships between the Alpine orogeny Fax: c41-31-6314843 and the Molasse Basin evolution on a transect across the central Swiss Molasse Basin. D. W. Burbank Department of Geosciences, Pennsylvania State University, The aim of this study is to reconstruct the deposi- University Park, PA 16802, USA tional and structural evolution of the Molasse of M. Mange eastern Switzerland. In particular, we use high-resolu- Department of Earth Sciences, University of Oxford, tion magnetostratigraphy to tie the sedimentary, struc- Parks Road, Oxford, OX1 3PR, England tural, and geometrical evolution of the proximal 254

Fig. 1 Geological bedrock Hegau map of the Swiss Molasse Ma NS Volcanics Basin and schematic strati- 14 Serra. OSM II graphy and age of the eastern Lg. Lake Constance Molasse and North Helvetic 18 I OMM Basel Tabular Jura Flysch (NHF) in a III Burdigal.

Miocene Zürich north–south cross section. II Folded Jura USM I–III and OSM I–II 22 Aquitan. refer to informal sub-units USM (see text). The study area is 26 N a p p e s I outlined Chattian 30

Oligocene Bern

Rupelian UMM 34 N NHF Eoc. Priabon. 50 km Siltstones Conglomerates Marls Sandstones Study Area Lake Geneva (Figs. 2, 3) A l p i n e Upper Freshwater Molasse (OSM) Folded Jura Crystalline Cores Upper Marine Molasse (OMM) Genève Helvetic Tertiary Lower Freshwater Molasse (USM) Lower Oligocene of Lower Marine Molasse (UMM)

Molasse Switzerlandbasin the Rhine Graben and Flysch

modified after KELLER et al. (1990) and SCHLUNEGGER et al. (1996)

Molasse. Different dispersal systems and their catch- gression and consists of shallow-marine sand- and silt- ment areas were identified using heavy mineral suites stones of the OMM interfingering with large fan deltas of sandstones and the clast composition of conglomer- at the southern basin margin (Keller 1989; Schaad et al. ates. The geometry and the location of these dispersal 1992; Bolliger et al. 1995), followed by fluvial clastics of systems in the basin were mapped to reconstruct shifts the OSM (Burdigalian to Serravallian). These lithostra- of depositional systems. The combined information tigraphic units, however, are strongly heterochronous about the stratal architecture in the basin and the struc- both from west to east and from south to north (Diem tural evolution in the source terrain allows an improved 1986; Keller 1989; Schlunegger et al. 1996). understanding about the relationships between Following Schlanke (1974) and Kempf et al. (1997), thrusting and exhumation in the Alps and the sedimen- the Molasse units of eastern Switzerland are informally tary processes in the Molasse Basin. subdivided into subunits that are based on the heavy mineral content of the sandstones and, if obtainable, the clast composition of the conglomerates: USM I Geological setting (Rupelian-Chattian), USM II (Aquitanian), USM III (lower Burdigalian), OSM I (Burdigalian, continental Five lithostratigraphic units classically subdivide the equivalent to the marine OMM), and OSM II North Alpine Foreland Basin (Matter et al. 1980; (Langhian-Serravallian; Fig. 1). The subunit USM III is Keller 1989; Sinclair et al. 1991) for which we use the only of local significance (eastern Swiss Molasse; German abbreviations; these are the North Helvetic Kempf et al. 1997) and corresponds temporally to the Flysch (NHF), the Lower Marine Molasse (UMM), the lowermost OMM of central Switzerland (basal Luzern Lower Freshwater Molasse (USM), the Upper Marine Formation of Keller 1989). Molasse (OMM), and the Upper Freshwater Molasse The study area is located in eastern Switzerland (OSM; Fig. 1). The Molasse deposits form two shal- between Lake Zurich and the Rhine valley and covers lowing-, coarsening-, and thickening-upward megase- the proximal part of the Molasse Basin, including the quences. The first megasequence consists of the deep Subalpine Molasse as well as the southern part of the marine NHF (Priabonian), the marine UMM (Rupe- Plateau Molasse (Figs. 1, 2). Here, the Molasse deposits lian) and the USM (Rupelian to lower Burdigalian), a are well exposed and provide excellent insight into the thick succession of fluvial clastics (Fig. 1). The Burdi- tectonic structure of the southern basin margin (Fig. 3). galian age of the youngest USM deposits in the study Two major thrust sheets make up the northern and area (Kempf et al. 1997) contrasts with the western part eastern part of the Subalpine Molasse (Kronberg and of the Swiss Molasse Basin, where the youngest Gäbris thrust sheets, Fig. 2C). The southern area is deposits of the USM comprise Aquitanian strata more disrupted and consists of two larger thrust sheets (20 Ma; Berger 1985; Schlunegger et al. 1996). The (Schorhüttenberg and Speer thrust sheets; Fig. 2C) and second megasequence starts with the Burdigalian trans- three smaller, internally disturbed thrust sheets 255

(Habicht 1945a). The northward dip of the upwarped derived directly from the thrust front (Habicht 1945a). strata of the Plateau Molasse is interpreted to be We add a marine depositional system that comprises caused by a south-vergent backthrust as a consequence all marine deposits of UMM and OMM (Fig. 2B). of imbrication of the southward-dipping thrust sheets These deposits include offshore marls and coastal sand- of the Subalpine Molasse (triangle zone; Stäuble and stones of the UMM (Frei 1979; Diem 1986) as well as Pfiffner 1991). shallow-marine silt- and sandstones of the OMM (Keller 1989).

Methods Sedimentary petrography Depositional systems For the identification of different dispersal systems, Based on detailed sedimentological logging in the sandstones were sampled along the magnetostrati- Subalpine Molasse (USM; Kempf 1998) and the OSM graphic sections and analyzed for the heavy mineral lithofacies classification of Bürgisser (1981), five depo- composition according to Füchtbauer (1958, 1964) and sitional systems define the megascopic architecture of Matter (1964). Data published by Hofmann (1957) and the alluvial Molasse deposits (USM, OSM) according Füchtbauer (1964) were included to complete the to Schlunegger et al. (1997c; Fig. 2B). petrographic record. We quantitatively studied the clast Floodplain depositional systems commonly consist composition of the conglomerates in order to distin- of laminated to massive mudstones, often showing guish between different alluvial fans and to recognize pedogenic features of high maturity (root traces, red- their catchment areas, including data from Renz violet mottling), abundant caliche nodules, and fine- to (1937a) and Habicht (1945a). For the quantitative clast medium-grained sandstones, that are interpreted as analysis the reader is referred to Schlunegger et al. crevasse splays or channels. Massive sandstone chan- (1993). nels are of minor importance. Sandstone channel-belt depositional systems comprise single or stacked medium-to-coarse channe- Palinspastic restoration lized sandstone bodies of 2 to 115 m thickness (sandstone/siltstone facies association [H3] of Bürgisser To understand the paleogeographic evolution of the 1981). They show an erosive base with pebble lags and southern basin margin, the allochthonous units of the rip-up clasts. These alternate with floodplain deposits Molasse had to be restored palinspastically to their that show less mature pedogenic features. position during deposition. The restoration of the Conglomerate channel-belt depositional system in a Subalpine Molasse of eastern Switzerland is largely more proximal position consists of 12-m-thick based on the stratigraphic and structural interpretation conglomerate beds interbedded with mature floodplain of a seismic line (Stäuble and Pfiffner 1991; Pfiffner et fines (conglomerate/siltstone facies association [H2] of al. 1997a) in combination with outcrop data. These data Bürgisser 1981). The conglomerates form amalgamated suggest a shortening of ca. 28 km, the presence of a units up to 20 m thick that can be traced laterally for stack of imbricate thrust sheets that flatten towards the several hundred meters. The clast size of the conglom- south, and a probable N–S extension of USM deposits erates is usually less than 20 cm. Alluvial megafan beneath the Alpine nappes (Fig. 3A). Moreover, the depositional systems (conglomerate facies association presence of a triangle zone is evident from seismic data [H1] of Bürgisser 1981) consist almost entirely of (Stäuble and Pfiffner 1991). Other interpreted seismic massive, crudely horizontally bedded conglomerates lines constrained by borehole data (Central Switzer- with a clast size up to 60 cm. Finer-grained sediments land: Vollmayr and Wendt 1987; Southern Germany are only rarely preserved as thin, laterally restricted and Austria: Müller et al. 1988; Schwerd et al. 1995; units. Occasionally mass flows are present in the most Vollmayr and Jäger 1995) show a similar tectonic struc- proximal facies. ture of the Subalpine Molasse. Tectonic interpretations Bajada depositional systems occur locally on the based on balanced cross sections lead to a similar flanks and on top of the alluvial megafan conglomer- amount of shortening of the Subalpine Molasse (Burk- ates. They consist of thick, fine-grained massive debris hard 1990; Pfiffner et al. 1997a). Due to lateral changes flows and narrow, ribbon-like channels with a chaotic of the tectonic structure, the palinspastic restoration in conglomerate fill and deep scours. The conglomerates the western part of the study area may not apply to the exhibit outsized clasts of more than 70 cm. They were entire study area (Fig. 3C). presumably deposited by hyperconcentrated flows. Sandstone bodies are subordinate. The conglomerates are also amalgamated to larger channelized bodies and Magnetostratigraphy may even form a small 3- to 4-km-wide independent fan. The bajada deposits occur only locally and their Nine sections were sampled for magnetostratigraphy in petrographic composition indicates that they are the Subalpine Molasse and Plateau Molasse of eastern 256

A St. Gallen

Lake Zürich

B St. Gallen

Lake Zürich 257

O identify growth of magnetic minerals during sample Fig. 2 A Tectono-stratigraphic map of the study area (modified processing. We identified a stable direction of the after Kempf et al. 1997) and locations of the magnetostratigraphic demagnetization vector between 200 and 350 7C that is sections. Marked sections are discussed in Kempf et al. (1997). B Facies map showing the distribution of the depositional systems interpreted as the characteristic remanent magnetiza- in the study area. Palaeocurrent directions (small arrows) indicate tion (Butler 1992). Therefore, we demagnetized and radial (fan-shaped geometry) and basin-axial drainage systems. C measured the remaining samples at three heating steps Tectonic map of the eastern Swiss Subalpine Molasse. The Schor- between 2007 and 350 7C. Fischer statistics (Fisher 1953) hüttenberg Thrust Sheet (1) and the Speer Thrust Sheet (2) are part of the Speer Thrust Zone. The maps are based mainly on were used to test the coherency of the magnetic direc- data from Saxer (1938), Renz (1937a, 1937b), Habicht (1945a, tions for each site. Groups of three samples were clas- 1945b), Ludwig et al. (1949), Büchi (1950), Ochsner (1975) and sified “class I” if k610 and “class II” if k~10, or two Bürgisser (1981) samples yielded an unambiguous polarity with k610. The mean magnetic vector of each site was used to calculate the virtual geomagnetic pole (VGP; Fisher Switzerland (Fig. 2A) comprising 586 sites and a total 1953) including an a95 error envelope for each VGP measured thickness of ca. 12.5 km. Four to six oriented latitude. Kempf et al. (1997) give a detailed description samples, predominantly mudstones and rarely fine- on the paleomagnetic processing and discuss the grained sandstones, were collected from each site. The magnetostratigraphy of the Hörnli, Jona, Goldinger demagnetization behavior for representative samples of Tobel, and Gäbris-Sommersberg sections. The locali- each analyzed section was identified in a pilot study by ties of the individual magnetostratigraphic sections are thermal and alternating field demagnetization. The shown on Fig. 2A. For more detailed information about samples were taken from various stratigraphic levels the paleomagnetic data and location of the sites the and depositional systems (see Schlunegger et al. 1996 reader is referred to Kempf (1998). for justification) and analyzed using a cryogenic magne- tometer. Thermal demagnetization was carried out in steps of 50 7C from room temperature to 550 7C, and in Results steps of 30 7C from 5507 to 640 7C. After each temperature step, the magnetic susceptibility was measured to Magnetostratigraphic framework Magnetostratigraphic calibration of micro-mammal Fig. 3 A Cross section through the eastern Swiss Subalpine faunas Molasse. B Palinspastic restoration. The amount of shortening within the Subalpine Molasse is ca. 28 km. C Location of the Well-correlated magnetostratigraphic sections that seismic line of Pfiffner et al. (1997a) contain micro-mammal-bearing sites have been used to

A C

18 km Kronberg Gäbris Thrust Sheet NS 4 Wattwil Churfirsten 4 Thrust Sheet OSM (c2) 0 (b) 0 OMM (a) USM II Seismic Line (c1) USM I - 4 - 4 UMM Lake Zürich Mesozoic 1 Speer (km) (km) 2 Basement Thrust Zone - 8 - 8 Alpine Nappes Lake Walen

0 10 km redrawn after PFIFFNER et. al. (1997) 1 Schorhüttenberg Thrust Sheet 2 Speer Thrust Sheet

B ca. 46 km N S 4 Wattwil Churfirsten 4

> shortening ca. 28 km 0 0

- 4 - 4 (km) Future Future (km) Triangle Zone - 8 Kronberg Thrust - 8 (a) Sheet Future (b) Schorhüttenberg Thrust Sheet Future (c1) Speer Thrust Sheet 0 10 km (c2) 258 establish a detailed bio-chronostratigraphic framework section (Fig. 2A,B; Bürgisser 1984) is assigned an age of (Burbank et al. 1992: two sections; Schlunegger et al. ca. 16 Ma based on the MPS. The Hörnli MPS (OSM 1996: eight sections; Kempf et al. 1997: four sections). II) comprises chrons 5ADn-5Ar.3r? spanning the time The magnetostratigraphic correlation of the sections period between ca. 14.5 and 13 Ma. from the eastern Swiss Molasse Basin (Gäbris- Sommersberg, Goldinger Tobel, Hörnli, and Jona section) is given in Fig. 4. These four sections, together Magnetostratigraphic correlation based on with two sections from central Switzerland (see details micro-mammal faunas in Kempf et al. 1997), were used to calibrate micro- mammal faunas of MN 3 to MN 7/8 (21–13 Ma; Fig. 4). The Sitter/OMM section shows an interfingering of The magnetic polarity stratigraphy (MPS) of the marine (OMM) and continental (OSM I) deposits at Gäbris-Sommersberg section (USM II–III) is inter- the eastern margin of the Hörnli fan delta (Figs. 2A, preted to encompass chrons 6AAn-6n of the global 5A; Keller 1989). Three reversals were detected in this Magnetic Polarity Timescale (MPTS; Cande and Kent 450-m-thick section defined by two or more class-1 1992, 1995) and represents the time interval between 22 sites. Only the lowermost reversal (N1) is represented and 19 Ma. This indicates that terrestrial USM deposi- by a single class-1 site. Sample spacing is 20–25 m on tion persisted in eastern Switzerland until ca. 19 Ma, average. The magnetostratigraphic section is whereas marine sedimentation (OMM) had already constrained by two micro-mammal sites taken from started in central Switzerland at ca. 20 Ma (Berger 1985; Schlunegger et al. 1996). The MPS of the Goldinger Tobel section (USM II to OSM I) is inter- Fig. 4 Magnetostratigraphic calibration of micro-mammal faunas preted to comprise chrons 6AAr.1r-5En, covering the from the eastern Swiss Molasse Basin, modified after Kempf et al. time interval from ca. 22 to 18.3 Ma. The Jona MPS (1997). The correlation of the individual magnetic polarity stratigraphy (MPS) including the micro-mamma faunas is constrained (OSM II) includes chrons 5Cn.3n?-5ABr covering a by the correlated Küsnacht bentonite (14.91B0.09 Ma; Gubler et time span of ca. 16.5–13.5 Ma. The stratigraphically al. 1992) following Bolliger (1992, 1994). Additional sections from important Hüllistein marker horizon located within this the central Swiss Molasse are not shown

Magnetic Polarity MN-levels Magnetostratigraphic calibration Timescale (MPTS) and reference faunas of micro-mammal faunas

[Ma] 5Ar.1 n r Hörnli 5Ar 5Ar.2r / n 5Ar.3r MPS 13 5AAn Jona 5AAr ? Grat ? MN 7/8 5ABn MN 7/8 MN 7/8 MPS 5ABr MN 7/8 MN 6-7 MN 6 5ACn ? Bachtel-Ornberg MN 6 MN 6 14 5ACr

Serravallian 5ADn 5ADr Frohberg (upper) 1 up. MN 5 n m. MN 5 5Bn.1 r 15 5Bn 5Bn.2n (middle) m. MN 5 Tobel Hombrechtikon 5Br MN 5 2 lo. MN 5 16

Langhian n 5Cn.1 r Vermes 1 (lower) 5Cn n 5Cn.2 r 5Cn.3n MN 4 MN 4 17 5Cr Tägernaustrasse MN 4 5Dn

5Dr Miocene 18 Trub-Sältenbach (upper) 5En b (lower) MN 3 5Er Goldinger Tobel 8 lo. MN 3b Burdigalian 19 MN 3 MN 3 6n Bierkeller (upper) a lo. MN 3a 20 (lower) 6r Goldinger Tobel 1 n 6An.1 r Vully 1 MN 3a * 21 6An b 6An.2n La Mèbre 698 6Ar La Chaux 7 MN 2 6AAn 6AAr.1 r a 22 n

Aquitanian 6AAr Les Bergières 6AAr.2 r 6AAr.3r n Goldinger Tobel CANDE & KENT (1992, 1995) after KEMPF et al. (1997) and MPS and BERGGREN et al. (1995) SCHLUNEGGER et al. (1996) 200 m Gäbris-Sommersberg MPS

1: Küsnacht bentonite (14.91± 0.09 Ma) 2: Hüllistein marker horizon *) KÄLIN 1998 (pers. comm.) 259 A Sitter/OMM Section Lithostratigraphy VGP-Latitude Sitter/OMM Conglomerate ° ° ° MPS (m) Sites -90 0 +90 Sandstone 400 2 2 Mudstone 1 1 N2 Class I Site R1 0 N1 Class II Site (with error on latitude) OMM OSM I Micro-mammal Site Lithostratigraphy and Biostratigraphy after KELLER (1989) A Sitter/OMM Section KELLER (1989), KÄLIN (1997): B 1 Goldach-Martinsbrücke (Trub-Sältenbach/upper MN 3b) 2 Sitter 418.5 m (Tägernaustrasse/MN 4 or younger)# Necker Section B VGP-Latitude Necker (m) Necker Section -90° 0° +90° MPS MöDDEN (pers. comm. 1994): Sites 3 OK-5 (MP 26 ± 1 zone) 4 OK-1 (MP 28 ± 1 zone) R14 4000 FREI (1979), ENGESSER (1990) (projected): 5 Ebnat-Kappel (Fornant 6/up.MP 28) 6 Wintersberg-Trempel (Küttigen or older/up.MP 29 or older) N14 7 Krummenau-Thur (Brochene Fluh 53/up.MP 30) 8 Krummenau-Umgehungsstrasse (Fornant 11/up.MN 1)

3500 R13 N13 R12 N12 R11 N11 3000 R10 8 8 N10

R9 N9 R8 7 7 N8 2500 R7 6 6 N7

5 5 2000 N6 R5 4 4 N5 R4 1500 N4 Fig. 5 A Magnetostratigraphy and micro-mammal R3 faunas of the Sitter/OMM section. The stratigraphic 1000 column shows the interfingering of marine (OMM) N3 and continental (OSM I) deposits. B Sedimentology, magnetostratigraphy and micro-mammal faunas of the Necker section (USM I–II). The reversals N1 and R1 3 3 R2 are not used for correlation because of multiple 500 N2 thrusts with unknown displacement. Small filled dots indicate the stratigraphic position of sampling sites. R1 Quoted error is the a-95 error on the VGP latitude. Class-I sites have three or more samples with Fisher N1 (1953) k110. Class-II sites have unambiguous 0 polarity, but k~10

Keller (1989; Fig. 5). Site 1 (Goldach-Martinsbrücke), younger (Fig. 5A; Keller 1989). Based on the calibra- well correlatable from approximately 3–4 km further tion chart of Kempf et al. (1997), these two mammal east to the section below the final marine transgression, sites allow the following correlation (Fig. 6): N2 is split contains a fauna of upper MN 3b (Kälin 1997), whereas into two segments and correlated to chrons 5En and the fauna of site 2 (Sitter 418.5 m) is of MN 4 or 5Dn, N1 and R1 represent the top of chron 6n and 260 MPTS MN-levels and reference faunas

(Ma) 5Ar.1 n r 5Ar 5Ar.2r / n 5Ar.3r 13 5AAn 5AAr ? Grat 5ABn MN 7/8 5ABr 5ACn ? Bachtel-Ornberg MN 6 14 5ACr

Serravallian 5ADn

5ADr Frohberg (upper) n 5Bn.1 5Bn r 15 5Bn.2n Tobel (middle) MN 5 5Br Hombrechtikon Sitter/OMM 16

Langhian n (lower) MPS 5Cn.1 r Vermes 1 5Cn n 5Cn.2 r 5Cn.3n ? MN 4 or younger 17 5Cr Tägernaustrasse MN 4 up. MN 3b 5Dn ? N2 5Dr Trub- 18 (upper) Sältenbach R1 R14 5En b N1 ?

Miocene Goldinger (lower) 5Er Tobel 8

Burdigalian 19 MN 3 (upper) 6n Bierkeller N14 a 20 Goldinger (lower) 6r Tobel 1 n R13 6An.1 r Vully 1 21 6An b N13 6An.2n La Mèbre 698 R12 6Ar MN 2 N12 6AAn La Chaux 7 22 r R11 6AAr.1 a R10 N11 6AAr n Les Bergières 6AAr.2 n r 6AAr.3r 6Bn.1 r n up. MN 1

Aquitanian 6Bn N10 23 6Bn.2n Fornant 11 MN 1 6Br R9 n Boudry 2 6Cn.1 r N9 6Cn n Brochene Fluh 53 6Cn.2 R8 24 r Küttigen MP 30 6Cn.3n N8 up. MP 30 Brochene Fluh 19/20 R7 6Cr Rickenbach MP 29 lo. MP 30 or older n Fornant 6 N7 7n.1 r 7n 7n.2n Fornant 7 MP 28

late Chatt. late 25 7r Boningen R6 7An 7Ar Wynau 1 MP 27 26 8n.1 r n up. MP 28 8n 8n.2n N6 Mümliswil-Hardberg 8r R5 27 MP 26 Oensingen N5 MP 28 9n ± 1 zone

early Chattian R4

Oligocene 28 Bumbach 1 9r

10n.1 n MP 25 10n r 10n.2n Talent 7 N4

29 10r Grenchen 1 11n.1 n MP 24 11n r R3 30 11n.2n 11r Rupelian N3 12n 31 modified after R2 MP 26 CANDE & KENT (1992, 1995) SCHLUNEGGER et al. (1996) ± 1 zone and BERGGREN et al. (1995) and KEMPF et al. (1997) N2

? R1

Fig. 6 Magnetostratigraphic correlation of the Sitter/OMM MPS and the Necker MPS to the Magnetic Polarity Timescale (MPTS) Necker MPS N1 of Cande and Kent (1992, 1995) based on micro-mammal biostratigraphy (Frei 1979; Engesser 1990). An error bar (B75 m) is added to the projected sites in the Necker section (see text) 261 chron 5Er, respectively, resulting in an erosional gap of either defined by only a single class-I or class-II site ca. 0.5 Ma within the OMM (covering at least chron (N7, R10, and N11). This assumption seems to be 5Dr). This correlation is justified by the evolutionary appropriate since the duration of individual channel stage of the upper MN 3b fauna, which is placed near belts are of the order of 104–105 years at most (Johnson the base of Trub-Sältenbach and interpreted to be et al. 1985; Miall 1991), whereas the duration of major slightly older than the reference fauna (Kälin 1997; D. magnetozones generally exceeds 105 years. Kälin, pers. commun.). Given the faunal data, the The projected micro-mammal faunas allow a good theoretically possible correlation of N1–N2 to chrons correlation of the magnetozones R6 to N10 to the 5En to 5Dn is very problematic. Alternatively, the MPTS (chrons 6Cr/?7n.1r to 6Bn.1n/6Bn.2n) within the upper segment of N2 represents the normal chrons of estimated error (Fig. 6). Below R6, the correlation of 5Cn, because the fauna of site 2 may also be younger the reversal patterns (N2–R5) is straightforward and than MN 4. The last correlation would, however, further constrained by two mammal faunas (although require a strong increase of the sedimentation rate less well defined). The upper part of the section because approximately 500 m of sediment (Büchi 1950) (R10–R14) is correlated by the succession of normal are present between N2 and the Hüllistein marker and reversed polarities to chrons 6AAr.3r-6An.1r that horizon at 16 Ma (Fig. 4). The existence of an erosional very likely have all been detected within the contin- gap within the OMM has already been suggested by uously prograding alluvial fan system (coarsening- and Büchi (1956) based on biostratigraphic data. In a thickening-upward sequence; Kempf 1998). No signifi- seismic study of the OMM of central Switzerland cant erosion of strata is recognized. Alternative correla- Schlunegger et al. (1997a) were able to identify magne- tions, e.g., N14 to chron 6n as suggested by the “long” tostratigraphically an erosional gap at the southern normal period, need to explain many missing reversals. basin margin within chron 5Dr. Hence, a correlation of Only the top (R14) is poorly defined and another the top of N2 to chron 5Dn seems to be more reason- possible correlation may include chrons 6An.1n and 6r. able and is thus used for further interpretations. As a This would, however, necessitate a major decrease in consequence, the basal marine transgression of the the sedimentation rate which appears inconsistent with OMM occurred ca. 1 Ma later than in central Switzer- the very proximal situation of the fan (Kempf 1998). land and reveals, again, the heterochroneity of facies in Hence, we correlate the Necker section with the chrons the Swiss Molasse Basin (see Keller 1989 and Schlun- 10n.1n to 6An.1r (N2–R14) representing the time egger et al. 1996 for justification). Nevertheless, other period between ca. 28.3 and 20.7 Ma (Fig. 6). Only a interpretations as mentioned above cannot completely few of the very short subchrons (^105 a) are either be ruled out. defined by only one class-I or class-II site (N7, R10, and The ca. 4300-m-thick Necker section is located in the N11) or were missed (8n.1r, ? 7n.1r, ? 7n.1n and Kronberg thrust sheet of the Subalpine Molasse 6Bn.1r). (Figs. 2A, 5B) and comprises most of the USM I–II. Two sites within the section contain micro-mammals (3 and 4 in Fig. 5B) representing the biozones MP 26 (B1 Magnetostratigraphic correlation based on petrography zone) and MP 28 (B1 zone). Four sites (5–8 in Fig. 5B) were projected from the Thur valley, approximately No micro-mammal faunas are present in the Thur, 4 km to the west, based on their stratigraphic position Steintal, and Sitter sections (Figs. 2A, 7). Their magne- relative to the first appearance of crystalline clasts in tostratigraphic correlation is therefore based on charac- the conglomerates which is supposed to be an isoch- teristic heavy minerals or key clasts in the conglomer- ronous reference level at the local scale (see also ates in comparison with well-constrained sections (see Fig. 9). The uncertainty of this reference level is esti- below). mated to B75 m at most, due to outcrop limitations in The Thur section is located in the southernmost the Thur valley. Speer thrust sheet and comprises the uppermost 70 m Good-to-excellent outcrop conditions in the Necker of UMM and 1400 m of lower USM I (Figs. 2A, 7A). section allowed a sampling density for magnetostrati- Sample spacing is 20–25 m with two gaps of ca. 100 m in graphy of ca. 25 m in the lower 1500 m and less than the upper part of the section. The Thur MPS consists of 20 m between 1500 and 3500 m (Fig. 5B). Coarse- 12 reversals defined by 50 sites (86% class I). The 1800- grained conglomerates prevented dense sampling in the m-thick Steintal section, located in the Schorhüttenberg uppermost part. The Necker section consists of 130 thrust sheet, is also composed of the uppermost UMM sites (76% class I) which define 28 magnetozones. The (130 m) and USM I (Figs. 2A, 7B). Sampling density is base of this section (N1 and R1) was probably affected 30 m on average; however, some larger gaps of more by thrusting and repetition of strata. Therefore, these than 100 m occur within the section. Nine reversals magnetozones were not used for magnetostratigraphic have been defined by 51 sites (73% class I). The correlation (Fig. 6). Due to the high sampling density, deposits of the Thur, Steintal, and lower Necker section we infer that all major chrons have been detected and (USM I) are characterized by the key heavy mineral are defined by two or more sites and by at least one spinel showing a peak of 620% and a drop thereafter class-I site. Only a few of the shorter magnetozones are (Frei 1979). This spinel peak provides a good correla- 262

Fig. 7 Sedimentology and A magnetostratigraphy of the Thur Section VGP-Latitude MPS A Thur section, B Steintal (m) section and C Sitter section -90° 0° +90° Class I Site 1500 Sites Class II Site (with error on latitude) N6 Conglomerate

R6 Sandstone 1000 N5 Mudstone R5 N4 R4 N3 R3

lower USM I N2 500 R2

N1 R1 0 UMM

B Steintal Section VGP-Latitude MPS -90° 0° +90° (m) Sites

1500 up. USM I

1000 R4

500 lower USM I N2 R2

N1 UMM R1 0

C Sitter Section VGP-Latitude MPS (m) -90° 0° +90° Sites

N8 1000 R8 USM II N7 R7 ? N6 R6 N5 500 R5 R4 N4 R3 N3 N2 R2 N1 upper USM I 0 R1 263 tion among these sections (Figs. 8, 9). Thus, we corre- Evolution of dispersal systems late magnetozones R6 (Thur MPS; Fig. 7A) and R4 (Steintal MPS; Fig. 7B) with R2 (Necker MPS; Fig. 6), The facies relationships at the southern basin margin which is correlated to chron 9r at ca. 28 Ma (Fig. 8). were delineated through detailed mapping of the Consequently, the Thur MPS is interpreted to correlate different depositional systems being heterochronous in to chrons 12 r-9n of the MPTS, representing an age of terms of the magnetic chronologies (Fig. 10). Five ca. 31.5–27 Ma (Fig. 8). The relatively short magneto- major dispersal systems were recognized in the study zones N1, N2, and N3 (compared with the MPTS), can area and were defined primarily by their characteristic be explained partly by erosion through scouring chan- suite of heavy minerals, but also by the clast composi- nels or outcrop gaps. The correlation of the Steintal tion of the conglomerates (Figs. 9, 11A). The geometry MPS is more problematic because of two larger and spatial distribution of the individual dispersal sampling gaps in N3 and R2 (Fig. 7B) where short systems is given by paleocurrent measurements subchrons, such as 11n.2n, 11n.1r, and 10n.1r, may have (Figs. 2B, 11B). been missed. If so, the Steintal MPS correlates with The oldest system, active between ca. 31.5 and chrons 12r–8r covering a time period of ca. 31–26.5 Ma 24 Ma, is referred to as Speer dispersal system (Figs. 9, (Fig. 8). This correlation is supported by the evolution 11A). It is supplied mainly by the Speer alluvial fan of the spinel showing an initial increase (in R2 of Thur with lesser contribution by the minor Stockberg alluvial and Steintal section) followed by a reduced abundance fan, a few kilometers to the east (Figs. 2B, 10; Habicht and the peak at ca. 28 Ma (Fig. 9). With these correla- 1945a, 1945b; Frei 1979). The conglomerates of both tions, the regression of the UMM sea started much alluvial fans are composed of sedimentary clasts earlier in the eastern Swiss Molasse Basin (Thur derived from Flysch and Austroalpine nappes (Renz section: 31.5 Ma; Steintal section: 31 Ma) than in 1937a). The sandstones are characterized by a high central Switzerland (30 Ma; Schlunegger et al. 1996). garnet content (Kempf 1998) and a suite of five heavy This reinforces the interpreted heterochroneity of the minerals, particularly zircon and spinel at the base, and lithostratigraphic units of the Molasse, as proposed by zircon and apatite higher up (Fig. 9). The Speer and Keller (1989), and implies the transport of large Stockberg fans revealed a cone-shaped geometry, indi- amounts of debris into the basin causing the coastline cating a transverse drainage that prograded into the to retreat. basin. Farther north the drainage pattern changed into The 1400-m-thick Sitter section, located in the a basin-axial orientation with a paleoflow toward the northernmost Gäbris thrust sheet, represents the more northeast (Figs. 2B, 10B). Fan progradation resulted in distal parts of the USM I–II (Figs. 2A, 7C). Sample an increasing sedimentation rate from ~0.3 mm/a to spacing is 20–25 m on average in the lower 700 m, and 10.5 mm/a between 31.5 and 25 Ma (Fig. 11B). 40 m between 700 and 1400 m including two larger The Speer system is succeeded by the more complex outcrop gaps. Seventeen magnetozones (R1–R9) were Kronberg-Gäbris dispersal system fed by three alluvial defined by at least two sites and include at least one fan systems (Figs. 10, 11A; Renz 1937a, 1937b; Habicht class-I site. There are, however, five single-point rever- 1945a, 1945b; Berli 1985): The Kronberg alluvial fan sals, most of them defined by class-I sites (Fig. 7C). (24–21 Ma) consists of a transverse alluvial megafan Due to a large sampling gap between N6 and R7, the and an axial conglomerate channel-belt depositional Sitter MPS was interrupted. A very important petro- system. The Gäbris alluvial fan (23–20 Ma) is repre- graphic marker is present, i.e., the occurrence of crys- sented by a distal conglomerate channel-belt deposi- talline clasts (gneiss) in the conglomerates (see details tional system, and the locally sourced Sommersberg fan in Fig. 9). This petrographic change is interpreted as (20–19 Ma) developed as bajada depositional system. the onset of a new dispersal system and therefore Between 24–20 Ma, the apex of these alluvial fans considered as isochronous with respect to the paleo- shifted 20–35 km in northeasterly direction (Figs. 2B, magnetic resolution. In the Necker (R8; Fig. 6) and in 10B). The conglomerates of the Kronberg and Gäbris the Sitter section (above N6; Fig. 7C) this marker is tied alluvial fans are characterized by the occurrence of to chron 6Cn.1r (Fig. 8). The base of the Sitter MPS felsic crystalline clasts (up to 20%) in addition to sedi- (R1) is correlated to chron 8r because its very low mentary clasts (Fig. 9) derived from the crystalline core spinel content does not allow a correlation with chron of the Austroalpine nappes according to Renz (1937a) 9r. The correlation between chrons 8r and 6Cn.1r is and Habicht (1945a). The crystalline clasts are the drawn tentatively due to the predominance of reversed major source of the key heavy mineral apatite in the and normal polarities (Fig. 8). N8 is correlated with the sandstones (Fig. 9; Füchtbauer 1964). Although occur- normal chrons of 6Bn; the upper limit may correspond ring at slightly different times, the substitution of to the reversals of chron 6AAr. Based on the petro- apatite by epidote took place in the youngest deposits graphy and on the reversal pattern we correlate the of both fans, where the first occurrence of greenschist Sitter section to chrons 8r to 6AAr.1r (ca. 27–22 Ma; and basic crystalline clasts in the conglomerates (~4%) Fig. 8). The resulting sedimentation rates (compacted) is also recorded (Fig. 9; Habicht 1945a; Hofmann 1957). are 0.2 mm/a for the distal USM I and 10.5 mm/a for These distinctive clasts are interpreted to have been the proximal USM II. derived from Penninic ophiolites and are the major 264 Age [Ma] MPTS

Mammal Hörnli MPS Biozonation 13 MN 7/8 ? MN 7/8 Jona MPS MN 6-7 MN 6 MN 6 MN 6 14

Serravallian up. MN 5 upper m. MN 5 15 1000 m lo. MN 5 middle

MN 5 16 Langhian lower MN 4 17 MN 4 ? ≥ MN 4 Gäbris-Sommersberg MPS up. MN 3b 18 upper

b MN 3 lo. MN 3b lower MN 3 Burdigalian 19 MN 3 ? Necker MPS Miocene upper Sitter/OMM MPS lo. MN 3a a MN 3a 20 lower Goldinger Tobel b 21 MPS MN 2 22 a

upper

Aquitanian ? 23 MN 1 lower up. MN 1 upper first appearance of crystalline clasts MP 30 lower 24 upper MP 29 lower upper MP 28 up. MP 30 late Chatt. 25 lower upper lo. MP 30 MP 27 or older ? 26 lower upper up. MP 28 ? 27 MP 26 lower

early Chattian 28 upper MP 28 MP 25 ± 1 zone lower Sitter MPS 29 Spinel-peak

Oligocene MP 24 30 Steintal MPS

Rupelian 31 Thur MPS MP 26 ± 1 zone 32

MPS's (Hörnli, Jona, Goldinger Tobel and Gäbris-Sommersberg) used to ? calibrate micro-mammal biozones (see KEMPF et al. 1997 for justification)

MPS's (Sitter/OMM and Necker) correlated by micro-mammal sites

MPS's (Sitter, Steintal and Thur) correlated by key clasts and heavy minerals with respect to the Necker section UMM 265

O this axial drainage was temporally limited to the time Fig. 8 Correlation of all magnetostratigraphic sections to the interval between 24 and 20 Ma (Fig. 11A), and sedi- Magnetic Polarity Timescale (MPTS). Simplified mammal mentation rates were nearly constant between 0.2 and biozonation after Schlunegger et al. (1996) and Kempf et al. (1997). For discussion see text 0.3 mm/a (Fig. 11B). The heavy mineral data suggest no mixing of this axial drainage with the radial Kronberg- Gäbris dispersal system (Figs. 9, 11A). The Hörnli dispersal system is the youngest system source for epidote (Renz 1937b; Füchtbauer 1964; of the eastern Swiss Molasse Basin and was active Dietrich 1969). Another source for epidote is crystal- during ca. 20–13 Ma (Fig. 11A; Kempf et al. 1997). The line clasts (gneisses and granites) associated with the heavy mineral composition of this system is generally greenschists (Füchtbauer 1964). Thus, it appears that dominated by epidote except at the base, where apatite, the catchment area of both fans evolved differently zircon, and rutile are major components (Fig. 9; Fücht- through time. Penninic crystalline units were eroded at bauer 1964). The clast composition is very similar to the ca. 21 Ma in the west and around 20 Ma in the east. Kronberg-Gäbris dispersal system, consisting of crystal- This is due either to differential uplift of Penninic units line clasts from Austroalpine and Penninic nappes in the hinterland, or to a different lateral (southward) (generally~20%, decreasing toward the top) and sedi- extension of the catchment areas. mentary clasts from Austroalpine, Penninic, and The presence of two petrographically very similar Helvetic nappes (Tanner 1944; Büchi 1950; Büchi and alluvial fans (Renz 1937b; Hofmann 1957) is indicated Welti 1951). The apex of the Hörnli alluvial fan is situ- by the cone-shaped geometries of the Kronberg ated 30 km west of the Sommersberg fan apex (western tributary) and Gäbris fan (eastern tributary), (Fig. 2B). Fan progradation to the north started slowly as well as by mapping of the zones of maximum accu- and increased shortly after deposition of the Hüllistein mulation of coarse conglomerates (Figs. 2B, 10B; marker bed at 16 Ma (Fig. 10A; Bürgisser 1980, 1981). Ludwig et al. 1949). Both fans, ca. 10–15 km apart, were Sedimentation rates of 0.2 to 10.3 mm/a were low active between 23 and 21 Ma and prograded in when compared with those of the Speer and Kronberg- northern direction (Fig. 10A). Sedimentation rates Gäbris fans. increased greatly during this period, from 0.3 to Large amounts of debris were delivered into the 11.0 mm/a in the Kronberg fan and from 0.3 to ca. OMM sea by the large Hörnli fan delta between 19 and 0.5 mm/a in the Gäbris fan (Fig. 11B). Bajadas devel- 17 Ma (Büchi 1950; Habicht 1987), as reflected in the oped on the flanks and on top of the Kronberg fan predominance of epidote in the terrestrial deposits (Fig. 10). The clast composition of these deposits is (Figs. 6, 10B). The heavy mineral association of the typically dominated by (Ultrahelvetic and North- intercalated marine sandstones is characterized by a penninic) Flysch clasts (Fig. 9; Gasser 1967; Woletz high amount of apatite typical of the OMM of eastern 1967). Forming on the eastern flank and succeeding the Switzerland (Figs. 5A, 9; Allen et al. 1985). The Gäbris fan (Fig. 10B), the Sommersberg fan shows a different heavy mineral composition at the base of the similar petrographic composition to the bajadas Hörnli dispersal system between 20 and 19.5 Ma encroaching the Kronberg fan implying the same local (Goldinger Tobel section; Fig. 9) possibly represents source from the frontal range of the Alps (Fig. 9). the last influence of an apatite-dominated hinterland of Nummulitic limestone clasts derived from the Eocene the former Kronberg-Gäbris dispersal system. Other Helvetic Einsiedeln, Bürgen, and Klimsenhorn forma- petrographic changes were recognized at 16 and 14 Ma tions (Yprésien-Lutétien; Menkveld-Gfeller 1997) in the sections of Jona and Hörnli, including a occur for the first time (Leupold et al. 1942; Tanner decreasing epidote content and an increase of a suite of 1944; U. Menkveld-Gfeller, pers. commun.). The accessory heavy minerals of predominantly meta- Gäbris-Sommersberg section contains individual beds morphic origin. The increase of staurolite indicates of Granitic Sandstone in the Gäbris fan derived from downcutting into metamorphic Penninic units, whereas the Höhrone and Napf dispersal systems (see below; topaz and andalusite suggest an influence from the Fig. 9; Renz 1937b; Füchtbauer 1964), suggesting the Bohemian Massif (Fig. 9; Hofmann 1976; Allen et al. interference of the different dispersal systems without 1985). mixing. The Höhrone and Napf dispersal systems (Fig. 11A) did not originate in the study area but deliver large Discussion amounts of debris through an axial drainage from SW into the region (Fig. 2B; Hofmann 1957; Schlanke 1974; Tectonic evolution of the southern basin margin Schlunegger et al. 1997b). These systems had been active between 24 and 14.5 Ma (Matter 1964; Keller 31.5–24 Ma. Initial stage: flysch nappe propagation 1989; Schlunegger et al. 1997b) and are characterized by the key heavy minerals zircon, apatite (Höhrone: During deposition of the UMM, the northern margin of Kleiber 1937; Schlanke 1974), and epidote (Fig. 9; the narrow seaway in the study area (Diem 1986; Napf: Füchtbauer 1964; Matter 1964). In the study area Berger 1996) seemed to be fault controlled since major 266

+ +

(%)

clasts

0 100

(%)

heavy minerals

0 100

K r o n b e r g A l l u v i a l F a n a F l a i v u l l A g r e b n o r K S p e e r A l l u v i a l F a n a F l a i v u l l A r e e p S local Bajada local

USM - I - USM Conglomerate Hüllistein Marker Bed Sandstone Mudstone II - USM

Necker

(#)

(%) (%)

clasts clasts

0 100 0 100 - * - - - -

Carbonate Nummulitic Limestone Flysch-Sandstone Felsic Crystalline Basic Crystalline

(%) (%)

Clasts:

heavy minerals heavy minerals

0 100 0 100 G G G

Gäbris AF Gäbris S p e e r A l l u v i a l F a n a F l a i v u l l A r e e p S Sommersberg LF Sommersberg

USM - III - USM USM-I USM - II - USM

Gäbris- Sommersberg UMM Steintal

r r r n n n n r n r n n r n r n n n n r

n r r n r r r r

5Ar.1 5Ar.2r / n 5Ar.3r 5Bn.1 5Bn.2n 5Cn.1 5Cn.2 5Cn.3n 6An.1 6An.2n 6AAr.1 6AAr.2 6AAr.3r 6Bn.1 6Bn.2n 6Cn.1 6Cn.2 6Cn.3n 7n.1 7n.2n 8n.1 8n.2n 10n.1 10n.2n 11n.1 11n.2n

5Ar 5AAn 5AAr 5ABn 5ABr 5ACn 5ACr 5ADn 5ADr 5Bn 5Br 5Cn 5Cr 5Dn 5Dr 5En 5Er 6n 6r 6An 6Ar 6AAn 6AAr 6Bn 6Br 6Cn 6Cr 7n 7r 7An 7Ar 8n 8r 9n 9r 10n 10r 11n 11r 12n 12r 13n

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Chattian Aquitanian Serravallian Burdigalian Langhian Rupelian

Oligocene CANDE & KENT (1992, 1995) and BERGGREN et al. (1995) Miocene

Magnetic Polarity Timescale (MPTS)

? ?

USM - II - USM OMM & OSM I OSM & OMM USM-I USM - I - USM OSM II OSM

UMM

Hörnli Sitter/OMM Thur

Sitter

S p e e r A F A r e e p S Hörnli AF Hörnli Gäbris AF Gäbris S p e e r A l l u v i a l F a n a F l a i v u l l A r e e p S Hörnli AF Hörnli Kronberg AF Kronberg

(%) (%) (%) (%)

heavy minerals heavy minerals heavy minerals heavy minerals

0 100 0 100 0 100 0 100 - - - -

OSM II OSM USM III USM USM - II - USM OSM I OSM

1000 m

Jona Goldinger Tobel

Hörnli AF Hörnli Hörnli AF Hörnli Napf AF Napf Höhrone AF Höhrone

Epidote Staurolite Apatite Zircon Tourmaline Hornblende Pyroxene Spinel Rutile Serpentine Sphene Kyanite Others

(%)

Heavy Minerals:

heavy minerals

0 100 0 100 ------267

O composition changed abruptly. In contrast, the clast Fig. 9 Heavy mineral and clast composition of all studied composition of the conglomerates remained unchanged sections and the relationship to the individual dispersal systems. until ca. 23.3 Ma, when the contribution of crystalline Samples of Granitic Sandstone in the Gäbris-Sommersberg section are marked G. Additional petrographic data from Renz clasts increased to approximately 15% (Fig. 9). This (1937b, #; Habicht 1945a, c; Hofmann (1957, *) and Füchtbauer discrepancy can best be explained by accretion of lower (1964, –) USM I to the orogenic wedge (Fig. 12C), such that reworking of these deposits contributed large amounts of sedimentary clasts to the newly formed crystalline- normal faults are present on seismic lines along the bearing Kronberg-Gäbris dispersal system and initially southern margin of the Plateau Molasse (Fig. 3A; overprinted the crystalline influence (Fig. 9). As Stäuble and Pfiffner 1991; Pfiffner et al. 1997a). Depo- another significant consequence of the incorporation of sition of the lower USM I (31.5–27 Ma; Fig. 9) started Molasse deposits (USM I) to the orogenic wedge, the with the northward prograding Speer alluvial fan. Its apex of the Speer alluvial fan was blocked off and the constant thickness in the Thur and Steintal sections apex of the new Kronberg alluvial fan shifted ca. 20 km over approximately 20 km in downcurrent direction farther northeast (Fig. 2B). A second apex (Gäbris allu- and constant dip from base to top (Fig. 2A) suggest vial fan) developed at 23 Ma approximately 12 km uniform subsidence during this initial stage of basin farther northeast (Fig. 2B). As a result of thrust activity evolution (Fig. 12A,B). No sediments were recognized within the Subalpine Molasse, the whole drainage younger than ca. 27 Ma in both sections (Fig. 10A). system migrated in a northeasterly direction. Farther north, the sedimentation rate increased in the At ca. 20 Ma more Molasse strata were accreted to Necker section between 27 and 25.5 Ma from 0.4 to the orogenic wedge (Fig. 12D). The changing clast 10.5 mm/a (Fig. 11B). Moreover, the amount of Flysch composition of the conglomerates of the Sommersberg clasts derived from the Alpine thrust front increased in fan implies a normal unroofing of the thrust front and the uppermost portion of the Steintal section at 27 Ma, the underlying Molasse (USM II) strata: The base is as well as in the Necker section between 26 and 25 Ma composed mainly of thrust-front material (Flysch sand- (Fig. 9; Habicht 1945a). This can be explained by a stones) towards the top, and the amount of clasts from phase of ongoing in-sequence thrusting of Flysch USM I and II deposits increased (carbonates and crys- nappes between 27 and 25 Ma that encompasses the talline up to 40%, Gäbris-Sommersberg section; Fig. 9). area of the Thur and Steintal sections. The tip of the This second phase of accretion influenced the drainage wedge at 25 Ma was south of the Necker section system considerably: The Kronberg and Gäbris alluvial (Fig. 12B). The advance rate of the Flysch nappes is fans were abandoned at ca. 21 and 20 Ma, respectively, roughly estimated to less than 3 cm/a considering our and the apex of the new local Sommersberg fan shifted chronologies and the interpreted N–S extension of the ca. 8 km to the northeast (Fig. 2B). lower USM-I deposits (Fig. 3A; Pfiffner et al. 1997a). The southern margin of the Plateau Molasse is made Increasing sedimentation rates (Necker section; up of deposits from the axial dispersal systems of Fig. 11B) and a gradual decrease of the dip angle up- Höhrone and Napf (Büchi 1950) that are located north section (50–357; Fig. 2A) suggest differential subsidence of the Kronberg-Gäbris dispersal system (Fig. 11A). after 26 Ma, due to extension of the tectonic load The palinspastic restoration (Fig. 3B) reveals a limited caused by basinward transport of the orogenic wedge N–S extent for the transverse Kronberg-Gäbris (see also Beaumont 1981; Sinclair et al. 1991). Hence, dispersal system (Fig. 12A). Its deposits were not after 26 Ma, the basin became more wedge-shaped at reported north of the Höhrone/Napf dispersal systems the tip of the thrust front. Between 25.5 and 24 Ma the (e.g., Büchi et al. 1965). Moreover, the distinct petro- decreasing sedimentation rate (Necker section; graphic compositions of these three dispersal systems Fig. 11B) reflects a time of relative tectonic quiescence remained unaffected by each other throughout deposi- in the wedge. A regional unconformity which devel- tion (Fig. 9). This suggests a rapid change of the paleo- oped during quiescence, as proposed by Schlunegger et flow direction of the Gäbris alluvial fan from transverse al. (1997b) ca. 50 km farther west, has not been recog- to axial within a few kilometers and a drainage divide nized at the thrust front during that time. that prevented these systems from mixing. A possible cause therefore is the beginning of deformation of the triangle zone (Fig. 12D). Active thrusting beneath the 24–19 Ma. Second stage: Molasse accretion to wedge, drainage area resulted in a fluvial divide along strike initiation of triangle zone separating the different dispersal systems between 24 At ca. 24 Ma older Molasse strata (USM I, and 20 Ma. The increased tectonic activity in northeas- 31.5–26.5 Ma) were incorporated into the Alpine orog- terly direction due to accretion and blind thrusting is enic wedge (Fig. 12C) as suggested by the following also a possible reason for the northwestern shift of the observations: the dip angle of USM-II deposits (ca. apex of the predominant Hörnli fan at ca. 20 Ma. A 24–21 Ma) decreases gradually from 35–207 up-section major change in the catchment area is evidenced by the (Necker section; Fig. 2A), possibly due to further modified petrographic composition of the new Hörnli increase of the tectonic load, and the heavy mineral dispersal system (Fig. 9). 268

N S

Ma Ma

(Pfänder Alluvial SW Fan) NE

Stock- berg Alluvial Fan 269

O to have been deposited at the southern basin margin. Fig. 10 A Chronological diagram (N–S) depicting the facies rela- Nevertheless, these sandstones are a possible source for tionships of the southern margin of the eastern Swiss Molasse andalusite and topaz in the Hörnli alluvial fan and thus Basin. B Chronological diagram (SW–NE) delineating the facies relationships in four cross sections through the major thrust units suggest that exhumation and subsequent erosion of the of the Subalpine Molasse (A–C) and the upwarped margin of the southern margin of the Plateau Molasse started at Plateau Molasse (D). The four cross sections are arranged from around 15 Ma, probably due to backthrusting. The south (A) to north (D) according to their palinspastically restored occurrence of andalusite and topaz in different strati- position. The magnetostratigraphic time frame is based on Fig. 8 graphic levels of the Hörnli alluvial fan (at 15 and 13 Ma) suggests uplift, erosion, and recycling of marine sandstones from the flanks of the Hörnli fan delta at 19–13 Ma. Third stage: backthrusting of Plateau different times. Molasse

The evolution of the Hörnli dispersal system started at Molasse Basin evolution and Alpine tectonics ca. 20 Ma when sedimentation on the Gäbris alluvial fan had almost finished (Fig. 10). At that time the The tectono-stratigraphic evolution at the Alpine thrust petrographic composition of both fans was very similar front is related to tectonic movements within the (Fig. 9). The sedimentation rate of the Hörnli dispersal Alpine orogen on a larger scale. Enhanced activity at system was low (0.2–0.3 mm/a) between 22 and 19 Ma the thrust front and marked changes in the catchment but increased up to 0.5 mm/a thereafter (Fig. 11B). This area of the dispersal systems represent major phases of coincides with the shift of the depocenter from the tectonic activity in the Alps. Based on our magnetostra- Subalpine realm to the area of the Plateau Molasse at tigraphic data from the eastern Swiss Molasse Basin, we 20–19 Ma when sedimentation in the Subalpine propose the following timing of tectonic phases: Molasse ended. A major erosional unconformity 1. At 31.5 Ma alluvial sedimentation started with the formed in the Plateau Molasse between ca. 18.3 and formation of the large Speer alluvial fan in the 17.8 Ma (Figs. 10, 11), as indicated by magneto- and southwestern part of the study area (Figs. 2B, 12A). biostratigraphic data from the eastern margin of the The high sediment input is a response to exhumation Hörnli fan delta (Sitter/OMM section; Figs. 5A, 8). of the southern Central Alps (Pfiffner and Hitz Bolliger et al. (1995) described a low-angle unconfor- 1997), which is linked to backthrusting movements mity between marine sandstones and underlying conti- along the Insubric Line associated with synmagmatic nental clastics at the southwestern flank of the Hörnli vertical movements during the Bergell intrusion at fan delta. The marine transgression is constrained by 32–30 Ma (Schmid et al. 1989, 1996; Blanckenburg both biostratigraphic data (Bolliger et al. 1995) and 1992). In addition, northward thrusting took place magnetostratigraphy (Goldinger Tobel section; Fig. 8). within the Helvetic nappes (initiation of Glarus An unconformity was also reported from the Molasse Thrust, Calanda phase of Milnes and Pfiffner 1977, Basin approximately 50 km further west between 18.1 1980; Pfiffner 1985, 1986). By that time Austroalpine and 17.7 Ma (Schlunegger et al. 1997a, 1997b). This nappes had been emplaced onto the Penninic units erosional phase possibly reflects uplift of the Subalpine and covered large areas of the Alpine realm, and Molasse (Fig. 12E) after accretion took place at around Penninic Flysch nappes formed the thrust front 20 Ma, probably in combination with a relative sea- (Pfiffner 1986). level drop (Keller 1989). Moreover, the tilted sedi- 2. Prograding Flysch nappes buried the proximal ments at the southern margin of the Plateau Molasse Molasse at ca. 27–26 Ma (Fig. 12B) and led to an show a decrease in dip angle from ca. 507 at the base increasing amount of Flysch clasts in the more distal (SE) to less than 107 on top (NW; Goldinger Tobel and area of the Speer alluvial fan and non-deposition on Jona section; Fig. 2A), suggesting continued syndeposi- the fan itself (Figs. 9, 10A). This northward propaga- tional uplift in the area of the triangle zone and Subal- tion of the orogenic wedge coincided with move- pine Molasse. The uplift is probably related to back- ments along the Glarus Thrust (Schmid 1975; thrusting of the southern margin of the Plateau Molasse Calanda phase of Milnes and Pfiffner 1977, 1980; (Fig. 12E). Andalusite and topaz, two characteristic Pfiffner 1985, 1986) and ongoing backthrusting heavy minerals of the Bohemian Massif, but unknown movements along the Insubric Line (32–19 Ma; from the Swiss Alpine hinterland (Büchi and Hofmann Hurford 1986; Schmid et al. 1987, 1997; Zingg and 1960; Hofmann 1976), occur rarely in the heavy mineral Hunziker 1990). Between 25.5 and 24 Ma sedimenta- assemblage of the Jona section at ca. 15 Ma and in the tion rates decreased from 10.6 to 0.4 mm/a (Necker Hörnli section at 13 Ma (~2% in “others”; Fig. 9). section; Fig. 11B) and coincided with a major uncon- These heavy minerals are only reported from the so- formity recognized in the central Swiss Molasse called Muschelsandsteine (OMM) of the northern Basin approximately 50 km further west (Schlun- Molasse Basin, transported by strong basin-axial tidal egger et al. 1997b), but not in the study area. The currents (Allen et al. 1985). Although lacking proof by decrease in sediment accumulation may reflect a heavy mineral analyses, similar sandstones also appear first phase of reduced tectonic activity. 270 A Plateau Molasse Subalpine Molasse N S

14 Paleocurrent patterns: Dispersal Systems: 14 OSM II Basin-axial Drainage Höhrone-Napf 16 (versus NE-E) 16 Transverse Drainage Local Bajada (versus NW-N) 18 OSM I H i a t u s Hörnli 18 OMM Kronberg-Gäbris USM III 20 Speer Dispersal 20 Marine 22 USM II 22 Magnetostratigraphic 24 24 Section

26 26 USM I 28 28

30 30

10 km 32 UMM 32 Ma Ma

B Plateau Molasse Subalpine Molasse N S Rates: 14 14 OSM II > 0.7 mm/a 16 ? 0.5-0.7 mm/a 16 0.3-0.5 mm/a OSM I 18 H i a t u s 18 OMM 0.2-0.3 mm/a USM III < 0.2 mm/a 20 20

22 USM II 22 Magnetostratigraphic 24 Section 24

26 26 USM I 28 28

30 30

10 km 32 UMM 32 Ma Ma 271

O enhanced exhumation and subsequent erosion Fig. 11 Chronostratigraphic schematic cross section (N–S) of the possibly due to backthrusting. The time interval of southern margin of the eastern Swiss Molasse Basin (based on backthrusting of the Plateau Molasse postdates Fig. 10A) showing A the distribution of the individual dispersal and drainage systems and B the sedimentation rates movements along the Insubric Line (Hurford 1986; (compacted) Schmid et al. 1996, 1997) and is in good agreement with Schlunegger et al. (1997b) who argued that backthrusting is associated by underplating and 3. Accretion of Molasse deposits to the orogenic wedge exhumation of the central Aar massif due to inden- is interpreted to have occurred at ca. 24 and 20 Ma tation of the Adriatic lower crust (Michalski and (Fig. 12C,D; refer to Schlunegger et al. 1997b who Soom 1990; Pfiffner et al. 1997b). also claimed accretion of earlier deposited Molasse strata to the orogenic wedge between 23 and 21.5 Ma in central Switzerland). This phase of Conclusion Molasse accretion coincided with enhanced uplift of the central Southern Alps along the Insubric Line High-resolution magnetostratigraphy of nine sections, after 26 Ma due to rapid cooling and exhumation in combination with mammal biostratigraphy, provides (110 km vertical uplift in the Bergell area; Hurford a very detailed chronology for the Molasse stratigraphy 1986; Giger and Hurford 1989; Zingg and Hunziker at the southern margin of the North Alpine Foreland 1990). Moreover, continued out-of-sequence Basin. This allows precise dating of the lateral and thrusting along the Glarus Thrust led to a passive vertical depositional evolution, sedimentation rates, northward transport of deformed Helvetic units and insights into the timing of tectonic processes that (Ruchi phase of Milnes and Pfiffner 1977, 1980; control the stratigraphic evolution. Pfiffner 1986), probably in combination with uplift Formation of petrographically different dispersal of the eastern Aar massif at ca. 20 Ma (Pfiffner et al. systems document the denudation history of the Alps. 1997b). Simultaneously, the petrographic composi- They were identified and defined by characteristic tion of the Molasse deposits changed markedly and heavy mineral and clast assemblages which mimic the imply major changes in the cachment area: At ca. composition of their former catchment areas. Changes 24 Ma basement units of the Austroalpine realm in the petrographic composition are therefore caused became initially eroded, and at ca. 21 Ma Penninic by modification of the catchment areas. basement was eroded as indicated by ophiolitic The thrust front was strongly modified in Late Olig- clasts in the Molasse (Dietrich 1969). Contrasting ocene to Middle Miocene times during phases of petrographic spectra (Fig. 9) point to a fluvial divide enhanced tectonic activity in the orogen leading to the separating the Höhrone/Napf and Kronberg-Gäbris growth of three prograding coarsening- and thickening- dispersal systems between 24 and 20 Ma (Fig. 12D). upward sequences of alluvial deposits. Propagation of Migration of the fan heads towards northeast the entire orogenic wedge initially started around between 24 and 20 Ma was probably caused by the 27–26 Ma probably in relation to uplift and exhumation shift of active Molasse thrusting in the same direc- of the southern Central Swiss Alps due to back- tion. Finally, the main drainage migrated back to the thrusting along the Insubric Line between 32 and west and the most recent Hörnli alluvial fan became 26 Ma and by movements along the Glarus thrust of the established. eastern Swiss Alps. Accretion of Molasse deposits to 4. An erosional unconformity is evident around 18 Ma the orogenic wedge took place in a second phase at ca. (Fig. 10). By that time backthrusting along the 24 and 20 Ma. This coincided with another period of Insubric Line (Hurford 1986; Schmid et al. 1989; rapid cooling, denudation, and inferred rock uplift of Zingg and Hunziker 1990) as well as movements the Central Swiss Alps along the Insubric Line after along the Glarus Thrust (Pfiffner 1986; Pfiffner et al. 26 Ma and continued out-of-sequence thrusting move- 1997b) had been finished. According to Schlunegger ments along the Glarus Thrust. Backthrusting of the et al. (1997b) this basin-wide erosional unconformity proximal foreland probably occurred thereafter and was produced by backthrusting of the southern was synchronous with uplift of the central Aar massif in Plateau Molasse due to the ongoing formation of the the course of underplating of the lower Adriatic crust. triangle zone. Thrusting near the deformation front and the geom- 5. Backthrusting of the southern margin of the Plateau etry of the thrust sheets themselves are also the most Molasse resulted in progressive rotation of the likely reasons for migration of alluvial fans. The deposits, which gradually flatten from base (S) to different alluvial fan heads shifted through time from top (N; Fig. 2A). The youngest deformed strata, ca. the southwest (Speer-Stockberg, 31.5–24 Ma) towards 15–14 Ma in age, mark the end of thrusting. the northeast (Kronberg, 24–21 Ma; Gäbris, 23–20 Ma; Reworking of OMM sandstones is suggested at 15 Sommersberg, 20–19 Ma) and then back towards the and 13 Ma by the occurrence of andalusite and topaz northwest (Hörnli, 20–13 Ma). A possible reason for in the heavy mineral assemblage. Sedimentation this migration was accretion of Molasse strata to the rates (Fig. 11B) increase at 15–13 Ma indicating orogenic wedge that forced abandonment of the former 272

A 273

O Büchi UP (1956) Zur Geologie der Oberen Meeresmolasse von Fig. 12A–E Tectonic evolution in five steps from 30–15 Ma. Full St. Gallen. Eclogae Geol Helv 48:257–321 arrows indicate active thrusting, open arrows depict no or only Büchi UP, Hofmann F (1960) Die Sedimentationsverhältnisse zur minor thrusting. See text for discussion Zeit der Muschelsandsteine und Grobkalke im Gebiet des Beckennordrandes der Oberen Meeresmolasse zwischen Aarau und Schaffhausen. Bull Ver schweiz Petrol Geol Ing 27:11–22 Büchi UP, Welti G (1951) Zur Geologie der südlichen mittellän- fan heads; hence, blocking of the drainage system at the dischen Molasse der Ostschweiz zwischen Goldingertobel und thrust front demanded a lateral escape of the feeding Toggenburg. Eclogae Geol Helv 44:183–206 rivers in four steps between 31.5 and 19 Ma. This Büchi UP, Wiener G, Hofmann F (1965) Neue Erkenntnisse im tectonically induced shifting is also reflected in the Molassebecken auf Grund von Erdöltiefbohrungen in der structure of the eastern Swiss Molasse: Different thrust Zentral- und Ostschweiz. Eclogae Geol Helv 58:87–108 Burbank DW, Engesser B, Matter A, Weidmann M (1992) sheets taper out laterally and are displaced from south- Magnetostratigraphic chronology, mammalian faunas, and west to northeast (Fig. 3). stratigraphic evolution of the Lower Freshwater Molasse, Haute-Savoie, France. Eclogae Geol Helv 85:399–431 Acknowledgements We thank F. Schlunegger (University of Bürgisser HM (1980) Zur Mittel-Miozänen Sedimentation im Jena) and O.A. Pfiffner (University of Bern) for fruitful discus- nordalpinen Molassebecken: das “Appenzellergranit”-Leit- sions. Special thanks go to P. Strunck (University of Bern) for niveau des Hörnli-Schuttfächers (OSM, Nordostschweiz). discussion and field support. The help of S. Burns (University of PhD thesis, ETH Zürich, 196 pp Bern) with the English is kindly acknowledged. We also thank Bürgisser HM (1981) Fazies und Paläohydrologie der Oberen H.-P. Bärtschi for technical assistance and H. Haas for the separa- Süsswassermolasse im Hörnli-Fächer (Nordostschweiz). tion of heavy minerals. S. Lund and S. Henyey (University of Eclogae Geol Helv 74:19–28 Southern California) provided much appreciated technical assist- Bürgisser HM (1984) A unique mass flow marker bed in a ance and advice with the paleomagnetic sample processing. The miocene streamflow molasse sequence, Switzerland. In: constructive reviews by J.-P. Berger and A. Wetzel improved the Koster EH, Steel RJ (eds) Sedimentology of gravels and manuscript and are kindly acknowledged. This paper is part of O. conglomerates. 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